1. Field of the Invention
This invention relates generally to a system and method for converting a power request signal for a fuel cell stack to a current set-point for the stack and, more particularly, to a system and method for converting a power request signal for a fuel cell stack to a current set-point for the stack that considers stack performance.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between the two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The stack controller needs to know the current/voltage relationship, referred to as a polarization curve, of the fuel cell stack to schedule stack reactant flows in accordance with power demands. The relationship between the voltage and the current of the stack is typically difficult to define because it is non-linear, and changes depending on many variables, including stack temperature, stack partial pressures and cathode and anode stoichiometries. Additionally, the relationship between the stack current and voltage changes as the stack degrades over time. Particularly, an older stack will have lower cell voltages, and will need to provide more current to meet the power demands than a new, non-degraded stack.
When the stack controller receives a power request signal from the vehicle operator, a control algorithm generates a current set-point for the fuel cell stack. In known systems, that current set-point is based on the stack polarization curve where it is assumed that the polarization curve is linear. Typically, only a single polarization curve is used for the fuel cell stack and that polarization curve is one that is determined for the beginning of life (BOL) of the fuel cell stack. However, as the stack ages, voltage degradation occurs in the stack where a higher stack voltage is required for a higher stack current. Thus, an increase in power request requires a higher current set-point. Further, external disturbances on the system cause voltage oscillations on the stack which translate to oscillations in the current set-point signal.